Youtube has a YT-Kids app which doesn't have ads.

It's handicapped in a lot of other ways. To name a few, you can't disable auto-play, if you want to disable recommendations the only way to do so is by selecting some channels that you would like your kid to have access to, and that will disable search AND casting. There's an un-skippable startup animation, and a whole lot of other animations that make the app feel slow. And the android tv version of it won't allow you to type in search queries with a keyboard (which the other youtube does allow).

In other words, it's better than the standard YT app in a lot of ways but it's an extremely frustrating UI for both the kids and adults.